Application Timing and Preharvest Intervals for Wheat Fungicides

Application Timing and Preharvest Intervals for Wheat Fungicides

July 2, 2008

Double Check the Restrictions for Your Fungicide

Figure 1. Septoria leaf blotch in wheat.

Figure 2. Fusarium head blight (scab) in wheat.

The 2008 growing season has been unusually wet especially in south central, eastern, and even southwestern Nebraska. The wet weather favored many diseases. The major diseases were Fusarium head blight (scab, Figure 1) and Septoria leaf blotch (Figure 2). Foliar and head diseases of wheat caused by fungi can be controlled by applying fungicides. However, proper timing of fungicide application is critical in achieving maximum efficacy and meeting legal requirements. This growing season there have been a few instances where fungicides apparently were applied later than they should have been. This article provides information on rates, application timing, number of sprays per season, and preharvest intervals for the most commonly used wheat fungicides in Nebraska (Table I).

To maximize efficacy of fungicides, it is essential that their application is timed properly according to label instructions. In wheat, as in other crops, fungicide application timing often is tied to crop growth stage. Timing also may be dependent on occurrence of environmental conditions favorable to disease development. Always adhere to label restrictions. In general, fungicides are most effective if applied before disease development or soon after disease detection. For suppression of Fusarium head blight (scab), fungicide should be applied at early flowering.

In wheat, fungicide application for optimal control of fungal foliar diseases usually is timed to protect the flag leaf. However, for early season diseases such as powdery mildew and tan spot, if conditions are favorable to disease development early in the season, it may be beneficial to apply a fungicide for early season disease suppression and follow up with a second application at flag leaf emergence. If a decision is made to apply two or more sprays, be careful not to exceed the maximum product rate for a single growing season. Also adhere to restrictions regarding the interval between two sprays and the pre-harvest interval.

Wheat Treated Too Close to Harvest

If wheat was treated with a fungicide which has a preharvest interval extending beyond the expected harvest date, wait as long as practical before harvesting. Postponing harvest allows more time for fungicide residue to degrade, which reduces grain contamination.

If you have questions, contact the Nebraska Department of Agriculture at 1-877-800-4080.